Furnace sight glasses (IR protection filters) are required in order to protect the human eye against the intensive radiation of hot bodies in the visible and in the infrared spectral area. The radiation of such bodies is increased in its intensity with the temperature of the bodies; the wave length is shifted with the rising temperature from the infrared spectral area into the visible one (0.4-0.8 .mu.m) (see FIG. 1).
For this reason, there is not just one furnace sight glass but, depending on the temperature of the radiating body, a glass with a certain light transmission degree to which a protective class is allocated.
Since infrared radiation can harm the eye (lens cloudiness), a minimum infrared absorption is specified for each protective class graded by medical points of view (see DIN 4647, Part 3). This standard is also a portion of the law covering technical working means (GTA).
For decades, two types of glasses have been industrially produced:
1. glasses dyed dark blue with cobalt and copper compounds which, however, do not have the minimum infrared absorption required according to DIN 4647, Part 3,
2. grey to green furnace sight glasses dyed with iron compounds which have the necessary infrared protection.